Notice the shape of the vectors in the 'Shape' option are different to the 'material' ones........ could this be the issue?
When driving on this particular area, the car spins into the air uncontrollably, having to sometimes reboot the game its so severe.

I can trigger this by creating a new project and making a straight road, adding an sobject like a fence (wooden fence or something of the likes with a few rectangular prism meshes) crossing that straight road somewhere (not too far from the start).
Then making it collidable and a wall material like cmwl > export.
Then load the track and drive flat out into the fence, you should get stuck and the game will go into freeze frame leaving you with only the option to go into task manager to kill it.

To fix it, go back to btb and make the fence drivable > export, load it up and drive flat out into it, you should bounce off it.

There's always a difference in how the program shows polygons in those two modes, but the only time I've seen such a marked difference is when I've modified the track (node/surface locations) after the terrain has been placed. The terrain attaches to the track at specific points which then get moved, wheres the remaining anchors stay in place.

This is my track, which doesn't have any terrain issues (ignore the two longitudinal lines in the second pic - my track is offset from the centre):

Attached Files:

Well, I tried selecting all meshes, they were all drivable and not collidable, so correct - nothing changed in game.
Deleted all meshes and drew new ones - still having problem.

Noticed that when putting new terrain in, with the click & drag option, the terrain appears some meters down the track and not at the point you want it to start - seems the anchor nodes have moved on the track and there is nothing I can do about it - I'm almost positive this is the problem. ? What to do ?

Banned

It worked for a while, now I see it's still doing this. When looking at the replay of the incedent, just before the car launches into the unknown, I see these 'Spline' types lines in the sky. Don't know where they are coming from, but they look something like this.

I recreated what they look like in Photoshop, so you get an idea. Now I'm not happy again.

I tried almost everything possible to rectify this. I removed the terrain connected to the track and put a wall in to drive on instead - still happens. I removed all grass objects and an SObject that was going across the track (removal completely takes property settings out the way) - still happens. I moved the material track anchors around - still happens.

So in my mind, its not material, object, shape, or properties that is causing this and I'm running out of options. Need to upload the track so you guys can see for yourself.